A conventional acoustooptic modulation device is described in Japanese Unexamined Patent Publication No. Syo 53-72,643, namely, 72,643/1978. As will later be described with reference to figures of the accompanying drawing, the acoustooptic modulation device comprises a single acoustooptic medium, a transducer attached to the acoustooptic medium, and an electric signal modulator connected to the transducer. The electric signal modulator amplitude modulates a plurality of carrier waves of different frequencies by electric signals to supply the transducer with modulated carrier waves, respectively. Acoustic or ultrasonic waves are propagated from the transducer to the acoustooptic medium in response to the modulated carrier waves and interact with an incident light beam which is given from an optical light source to the acoustooptic medium. As a result, the incident light beam is acoustooptically modulated by the acoustic waves into modulated light beams which are recorded on the recording medium as the information signals.
The acoustooptic modulation device is disadvantageous in that the modulated light beams are inevitably reduced in intensity, namely, luminous energy on account of an increase of a reflection loss or a variation of diffraction efficiency.
In the copending U.S. patent application Ser. No. 517,346 filed on July 26, 1983, by S. Amano et al, an acoustooptic modulation device is proposed wherein a plurality of transducers are attached to a single acoustooptic medium along a plurality of incident light beams, respectively. The transducers are activated by modulated electric signals to individually optically modulate the incident light beams, respectively. It should be noted here that each modulated electric signal is produced by modulating a single carrier wave of a common frequency and consequently falls within a common frequency band. With this structure, it is possible to reduce the reflection loss and to improve the diffraction efficiency insofar as each modulated signal falls within a common frequency band.
However, the reflection loss and the diffraction efficiency are adversely affected when each carrier frequency is varied over a wide frequency band.